The instant invention deals with a trimodal piston driven sash operator for fume hoods and fume hoods so equipped. Fume hoods are a common item of most every chemical laboratory and have been employed in one form or another for many years. As new materials and techniques are evolved, the fume hood undergoes modifications to attempt to ensure the working personnel freedom from spills and burns, noxious gases and chemicals. In recent years, this concern for safety has been coupled with a concern for energy conservation and certain modifications were made in standard fume hoods to ensure some energy savings while retaining the safety aspects of such hoods. For example, the air intake and exhaust ducts were adapted to electrical or penumatic dampers or adjustable speed motors which would work to adjust motor speeds or duct openings in order to utilize only the mass of air that was required for safety purposes while not allowing large volumes of air to continuously pass through the fume hood and cause a waste of energy.
Another recent development in energy conservation was the use of an automatic fume hood sash operator which was electrically coupled with an automatic switch mechanism, such as a rubber floor switch mat, or a photo electric eye such as disclosed in U.S. Pat. No. 4,150,606 issued Apr. 24, 1979 to Laurence N. Nelson,. The gist of that invention appears to be a sash operator which is actuated by a continuous cable loop carrying an activator switch which contacts limit switches which are built into the line of travel of the activator switch. A movable sash member of the hood structure is raised or lowered in correspondence to the movement of the activator switch. No provision is made in the Nelson invention for control of the sash member in a stop position if the sash member encounters an obstruction. Furthermore, the Nelson sash closer cannot be used in a fume hood where the counter-balancing weight for the sash member is required to be in the back of the unit. Finally, it is obvious that the Nelson invention would not be readily adaptable to existing fume hood structures since the design of the closure mechanism requires the counter-balancing weights to be positioned in the path of the control units for the hood, thus requiring extensive modification of hoods that are in place and functioning today.